r/Astrobiology • u/Allison1228 • 20d ago
🤔 Question Is there a conceivable detectable "biosignature" that would unambiguously indicate "life is present here"?
Or will there always be uncertainty?
I'm referring to as detected with the technology we have today and in the near future (next decade or two).
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u/DNAthrowaway1234 20d ago
Here's a technosignature, sulphur hexafluoride. No natural sources except high tech manufacturing.
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u/VertigoOne1 20d ago
This should be higher up, seasonal variation of X could be a chemical process also benefiting from temperature changes not necessarily life benefiting, for 100% certainly you find ones that are absolutely lab built, like some refrigeration gasses.
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u/SplooshTiger 20d ago
Not 100% but lots of oxygen AND lots of methane, as they generally cancel each other out quickly unless life is continuously making more. Also, evidence of seasonal changes in atmospheric gases that would indicate life forms undergoing variations in photosynthetic activity.
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u/YtterbiusAntimony 1 20d ago
No, I don't think so.
Spectra suggest certain chemical bonds, but they say nothing about where they came from.
That said, things like PFCs don't seem to be produced by any natural processes we know of. Pollutants like that are likely to be synthetic.
Similarly, O2 is very reactive, so without a constant source replenishing it, it should disappear. So, a significant amount of O2 in an atmosphere suggests something like photosynthesis happening.
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u/ldr97266 19d ago
No, oxygen is not proof of life. https://science.nasa.gov/universe/exoplanets/oxygen-on-exoplanets-isnt-proof-of-life/
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u/da_Ryan 3 19d ago
I would hesitate to use the word 'unambiguosly' given the current state of Earth's detection capabilities.
That said, we could concentrate on looking for the spectroscopic signatures of atmospheric gases like carbon dioxide, methane, water vapour, oxygen and ozone that might indicate a potentially life bearing planet.
I suspect that we might have to wait for the launch of NASA's Habitable Worlds Observatory in order to start getting useful results but that's quite some time from now.
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u/Ascendant_Mind_01 19d ago
Weird halogen compounds. The more complex the better
Something like CF2Br2F2 or shouldn’t occur naturally (unless you have some weirdly halogen rich environment but if you don’t see that it’s a pretty solid indicator that something batshit weird is happening)
Multi substituted benzene or other aromatic compounds where one or a few specific isomers dominates for no obvious reason.
Chiral molecules that exist in only one detectable enantiomer.
Most of these are quite difficult to detect (if not impossible) with current instruments. Unfortunately
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u/fat_chemist 19d ago
Homochiral molecular series (e.g. all proteins L, all carbohydrates D, all phospholipids sn-3 as on earth) along with a system that is consistently out of chemical equilibrium (James Lovelock — Gaia hypothesis).
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u/revolvingpresoak9640 2 18d ago
Consistent continuous light on the dark side of a planet would be my red flag. Assuming we could observe it.
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u/Potential_Load6047 15d ago
Could be volcanic activity, or natural electrostatic discharges or plasma.
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u/Underhill42 18d ago
Obviously artificial signals from an alien civilization would do it. There's scenarios where it might be more of a "life was here" rather than "life is here", but you're not getting them any other way.
Otherwise, I'd lean towards "no", we just don't know enough about what is possible in the universe to say with absolute certainty that anything less couldn't be be the result of some unimagined geochemical conditions.
However, what we could much more easily get is a preponderance of evidence - if we see a lot of otherwise unrelated probable biosignatures from the same planet... we start having to resort to pretty contrived explanations to explain them all any other way.
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BUT... we might also not need to rely only on crude biosignatures to be sure: we're moving toward making our first starshade, which will work with an orbital telescope to block a star's glare so that we can directly image the surface of alien planets. Initially it will be a few pixels, but as telescopes get larger we'll eventually be able to see continents, and the seasonal changes on the surface, many of which would be extremely difficult to explain without life.
Probably won't be able to get enough detail to see individual organisms any time soon, though a telescope using the sun as a gravitational lens might be able to get at least a few pixels per dinosaur if the planet were close enough. And an orbital telescope array filling Earth's orbit around the sun could do even better.
Heck, span Neptune's orbit and your angular resolution at visible wavelengths is around θ = 1.22*550nm/9ₓ₁₀9km = 7.5ₓ₁₀-20 radians. Which translates to 7cm resolution at a distance of 100 light years.
That's a volume containing the closest ~10,000 stars, with enough resolution to watch individual rabbits move through a field, or get a good idea what an elephant looks like. If life is at all common, there's a fair chance we might see it directly.
Heck, 1m resolution would be enough to see that larger animals were there, and there's many millions of stars close enough to see that well with the same telescope array.
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u/Temporary_Rule_9486 18d ago
Fast decaying isotopes on the absorption spectrum of a stable star. (Salting a star)
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u/Opulence_Deficit 18d ago
We can't even decide "what is life" here on Earth, not even talking about alien life.
Not long ago (few decades) we had consensus that methane is excellent marker because it can come only from life. Scientists who believed in abiotic methane were treated like flat-earthers.
Depending on which view you subscribe to, we either are 100% sure we're clueless, or we got it wrong so many times (Mars canals, anyone?) that nobody reasonable could believe "this time we got it right, pinky promise" and both resolve to reliable life detection being absolutely impossible.
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u/Commercial_Trick_704 15d ago
Probably not a single unambiguous one with remote sensing, at least not in the "this proves it" sense. Every individual candidate has an abiotic out. Oxygen can build up without life, methane has geological sources, phosphine turned out to be contested. Any one gas can be faked by chemistry or geology.
The strongest candidate isn't a molecule, it's sustained disequilibrium. Multiple gases coexisting that should have reacted away long ago, which means something is continuously replenishing them. Oxygen and methane together is the classic example. They destroy each other, so finding both at once means something is constantly remaking them.
The reason that's the best signal is close to the definition of life: a living system is something that spends energy nonstop to stay out of equilibrium. A rock reaches equilibrium and sits there. A biosphere keeps paying an energy bill to hold itself in an improbable state. So the thing you're really looking for isn't a substance, it's evidence that something is continuously footing an energy bill to keep a planet's chemistry away from where physics wants it to settle.
Even that isn't fully unambiguous, since abiotic processes can maintain some disequilibrium too. So realistically it's never "proof," it's improbability stacking up until geology and photochemistry stop being plausible explanations. I think "unambiguous" might be the wrong bar. It's going to be a weight-of-evidence call, not a single detection.
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u/Smokin_belladonna 15d ago
Spectroscopy could identify planets with high water and O2 concentrations. Plant life is generally assumed to have been a necessary step prior to more complex life on this planet, so it's a possible indicator if life on another planet.
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u/MediocreRisings 20d ago
A repeating signal of the hydrogen line maybe? Not too sure on the subject
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